Electroless plating of substrates

ABSTRACT

Electroless deposition of copper from solution onto a substrate, particularly a polymeric laminate for use in electronic circuitry, is accomplished by first treating said substrate with a stilbene-based fluorescent brightener to sensitize said substrate and thereafter contacting said substrate first with a solution of a catalytic metal and then with an electroless copper plating solution.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of Ser. No. 07/976,866 filed Nov. 16,1992 U.S. Pat. No. 5,292,557.

BACKGROUND OF THE INVENTION

The invention is related to the deposition of metals on substrates,particularly reinforced polymer laminates of the type used in electroniccircuitry.

Electronic circuits are typically formed on reinforced polymer laminatesby depositing metals, usually copper, on the surface. The circuitpatterns may be made by placing a thin layer of copper foil on thesurface of the laminate and then removing portions of the copper toleave the desired pattern. Another method is to deposit the copper froma solution, either as a continuous layer which is treated as justdescribed or preferably, only on areas which form the pattern.Electroplating is not feasible because the laminates are not electricalconductors, hence the method called "electroless plating" has beendeveloped. Solutions of copper salts are prepared and brought intocontact with the substrate. In order to deposit the metal from solutionit has heretofore been common practice to sensitize the surface of thesubstrate with a metal which is catalytic to the deposition of copper.Many metals have been suggested to have such capability, but in practicethe most commonly used sensitizer has been the combination of palladiumand tin salts in an acid solution. An early patent is U.S. Pat. No.3,011,920 which describes a colloidal solution of such metals. Anoptically clear, non-colloidal solution is the subject of U.S. Pat. No.3,682,671.

While metals are currently being used to catalyze the deposition ofcopper from solution, it will be evident that the use of a separatesensitizing solution involves additional steps and extra costsassociated with the metals, particularly the expensive palladium. If itwere possible to sensitize the surface of the substrate with lessexpensive materials, especially by incorporating such materials into thesubstrate, significant advantages would be obtained in the manufactureof electronic circuits on polymeric laminates and in other applicationswhere electroless deposition of copper is used.

SUMMARY OF THE INVENTION

An improved method of electroless deposition employs a stilbene-basedfluorescent brightener to sensitize the surface of a substrate.Electroless deposition of copper from solutions can be accomplishedwithout using catalytic metals such as palladium. Furthermore, when suchbrighteners are used with conventional electroless plating processeswhere palladium is used, it has been found that the rate of copperdeposition is increased and the effect of changes in plating bathcomposition is reduced.

In one aspect the invention is a reinforced polymer laminate,particularly a glass-reinforced epoxy laminate, which contains astilbene-based fluorescent brightener. The brightener may be added tothe polymer when the laminate is formed or may be applied to the surfacelater, either uniformly or in the pattern of the electronic circuitry.

A preferred fluorescent brightener has the CAS number 27344-06-5, whichhas the formula: ##STR1##

DESCRIPTION OF THE PREFERRED EMBODIMENTS Electroless Plating

As previously discussed, it is common practice to deposit layers ofcopper by electroless plating onto reinforced polymer laminates duringthe fabrication of multi-layer circuit boards. Electroless plating alsofinds other applications and the invention is not necessarily confinedonly to electronic circuit applications.

The electroless plating process usually involves two principal steps,namely, applying a metal (or metals) catalytic to the deposition ofcopper (or other metal such as nickel in other applications) and thencontacting the sensitized substrate containing the catalytic metal withanother solution to plate copper on the substrate.

Various metals have been suggested to be catalytic to the deposition ofmetals such as nickel, cobalt, chromium, and copper from solution. Forcopper deposition, U.S. Pat. No. 3,011,920 suggests Cu, Pb, Pt, Rh, Ru,Os, Ir, Fe, Co, C, He, Ni, Al, Au, Pd, and Me, although Au, Pt, and Pdwere preferred. Pd and Sn in an acid solution are believed to be stillin commercial use. Clearly, the use of a separate processing step whichconsumes expensive metals and which is essential to the successfuldeposition of copper is not desirable. If less expensive sensitizingmaterials could be found and if they could avoid the need for orsimplify the separate process step now required, the art would besignificantly advanced.

The copper solutions are formulated to provide compositions which areunstable in the presence of the sensitizing agents (e.g. Pd and Sn) andresult in the reduction of the copper (or other metal) ions to themetallic state so that they are precipitated on the surface where thesensitizing agents are located. Typical metal solutions are given in theexamples of U.S. Pat. No. 3,011,920. In the present invention, similarsolutions may be used or others familiar to those skilled in the art,provided that precipitation of metal from solution with the sensitizingagents of the invention occurs. Typically, a copper (or other metal)solution would be prepared and brought into contact with a substratecontaining or coated with a sensitizing agent of the invention for aperiod of time sufficient to deposit the desired thickness of metal.Alternatively, the substrate of the invention can be contacted withconventional sensitizing agents such as Pd and Sn before beingintroduced into the copper solution.

Sensitizing Agents

The sensitizing agents of the invention are found in the family ofstilbene-based fluorescent brighteners. These materials absorbultraviolet light and emit visible light and are used for variouspurposes, such as in textiles and as additives in inks and coatings.They may be included in epoxy or other polymer laminates to preventultraviolet light from penetrating the laminate and interfering with theuse of photoresists on the surface.

Typical members of this family of fluorescent brighteners are preparedfrom amino compounds of 2,2'-stilbene disulfonic acid. Many, includingthe preferred fluorescent brightener, react 4,4'-diamino-2-2'-stilbenesulfonic acid with cyanuric chloride and then further react the product.In particular, the preferred fluorescent brightener has the GAS No.27344-06-5, which has the following formula: ##STR2##

Substrates

The invention has potential uses in any substrates which are now orcould be plated by the electroless plating process. In particular,plastic articles and most particularly, reinforced polymer laminates areplated to provide conductive circuit paths on their surface. While epoxylaminates are most common, the invention Could be applied to suchpolymers as polyimides, cyanate esters, bismaleimide-triazines,phenolics, as the like. In laminates, reinforcement is typicallyprovided by woven and non-woven glass fibers (e.g. E-glass, S-glass),polyamides (e.g. Kevlar®), paper, quartz, and the like.

The concentration of the sensitizer at the surface of the laminateshould be in an amount effective to sensitize the electroless plating.For polymer laminates which incorporate the fluorescent brightener intothe polymer mixture used as the matrix for the laminate, this is about0.001 to 10 wt. %, preferably 0.1 to 2 wt. % based on the polymer.Alternatively, where no other purpose is to be accomplished by thefluorescent brightener, it may be applied to the surface of thesubstrate in a suitable composition and in amount adequate to sensitizethe surface for electroless plating. It will be clear that one advantageof the later procedure is that the sensitizer could be applied with amask defining the circuit pattern to be obtained. Thereafter, contactwith the copper solution would directly deposit the metal in the desiredpattern.

Example 1

A mixture suitable for coating glass fabric was prepared by combining anepoxy resin (Dow Chemical 71881) with 0.125 wt. % of Leuchophor BSB (CAS27344-06-5 Sandoz Chemical Co.), 2.8 wt. % dicyandiamide, and 0.044 wt.% 2-methyl imidazole (all based on epoxy resin solids) and solvent (53wt. % based on epoxy resin of DMF and acetone, weight ratio 0.77/1). Aglass reinforcing fabric (7628 fabric-CS718 finish Clark-Schwebel) wascoated with 43% by weight of the mixture. The coated fabric was thencured by heating to 175° C. under a pressure of 900 psig (6205 kPagauge) and holding for 65 minutes.

Example 2

The epoxy laminate of Example 1 was placed in an electroless platingbath containing a soluble copper compound. It was found that the copperplated onto the laminate and it was concluded that the fluorescentbrightener was functioning as a catalyst for the deposition of copper.

Example 3

An epoxy laminate was prepared in a similar manner to that of Example 1.An epoxy resin (Dow Chemical 71881) was combined with 0.125 wt. % ofLeuchophor BSB, 2.8 wt. % dicyandiamide, and 0.044 wt. % 2-methylimidazole (all based on epoxy resin solids) and solvent (38 wt. % basedon epoxy resin of DMF and acetone, weight ratio of 0.77/1). A glassreinforcing fabric (7628 fabric CS718 finish Clark-Schwebel) was coatedwith 44±2 wt. % of the epoxy resin mixture. The coated fabric was curedby heating to 175° C. under a pressure of 900 psig (6205 kPa gauge) andholding for 65 minutes.

Example 4

The cured epoxy laminate of Example 3 was tested in a laboratorysimulation of a commercial electroless copper deposition process. Inthis process the laminate was dipped into a bath containing a palladiumcatalyst before being introduced into the copper solution. When comparedwith a conventional FR-4 epoxy laminate the laminate containing thefluorescent brightener was found to deposit copper at a rate about25-30% higher and required less adjustment to the solution compositionas a series of tests were carried out. It was concluded that the use ofa fluorescent brightener had advantages where palladium catalysts wereused as sensitizing agents and thus could provide benefits whenconventional electroless plating processes were employed.

I claim:
 1. A method for electroless deposition of copper from solutiononto a substrate comprising treating said substrate with an effectiveamount of a stilbene-based fluorescent brightener to sensitize saidsubstrate and thereafter first contacting said substrate with a solutionof a catalytic metal and then with an electroless copper platingsolution.
 2. The method of claim 1 wherein said fluorescent brightenerhas the formula ##STR3##
 3. The method of claim 1 wherein saidfluorescent brightener is incorporated into the substrate.
 4. The methodof claim 1 wherein said fluorescent brightener is placed on the surfaceof the substrate.
 5. The method of claim 1 wherein said substrate is areinforced polymer laminate.
 6. The method of claim 5 wherein saidpolymer is selected from the group consisting of epoxy, polyimide,cyanate ester, bismaleimide-thiazine, and phenolic.
 7. The method ofclaim 6 wherein said polymer is epoxy.
 8. The method of claim 5 whereinsaid laminate is reinforced with a member of the group consisting ofwoven and non woven glass, polyamide, paper, and quartz.
 9. The methodof claim 1 wherein the substrate is a polymer laminate and theconcentration of said brightener is about 0.001 to 10 wt. % based on thepolymer.
 10. The method of claim 9 wherein said brightener concentrationis about 0.1 to 2 wt. %.
 11. An electroless plated substrate produced bythe method of claim 1.